Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

97
α AND β LINKS
The hydroxyl group on the carbon that carries the
aldehyde or ketone can rapidly change from one
position to the other. These two positions are
called α and β.
O
OH
OH
β hydroxyl
α hydroxyl
As soon as one sugar is linked to another, the α or
β form is frozen.
O
SUGAR DERIVATIVES
The hydroxyl groups of a simple
monosaccharide such as glucose
can be replaced by other groups.
For example,
CH HO
2 OH
O
OH
HO
OH
N-acetylglucosamine
H
NH
C O
CH 3
CH 2 OH
O
OH
H
NH 2
glucosamine
glucuronic acid
OH
O
HO
OH
C
O
OH
OH
OH
DISACCHARIDES
The carbon that carries the aldehyde
or the ketone can react with any
hydroxyl group on a second sugar
molecule to form a disaccharide.
The linkage is called a glycosidic
bond.
Three common disaccharides are
maltose (glucose + glucose)
lactose (galactose + glucose)
sucrose (glucose + fructose)
The reaction forming sucrose is
shown here.
α glucose CH 2 OH
O
OH
HO
OH
OH
CH 2 OH
O
OH
HO
OH
+
β fructose
HOCH 2 O
H O
H 2 O
HOCH 2
HO
CH 2 OH
OH
O
HO
O
CH 2 OH
OH
sucrose
OLIGOSACCHARIDES AND POLYSACCHARIDES
Large linear and branched molecules can be made from simple repeating sugar
subunits. Short chains are called oligosaccharides, while long chains are called
polysaccharides. Glycogen, for example, is a polysaccharide made entirely of
glucose units joined together.
branch points
glycogen
COMPLEX OLIGOSACCHARIDES
In many cases a sugar sequence
is nonrepetitive. Many different
molecules are possible. Such
complex oligosaccharides are
usually linked to proteins or to lipids,
as is this oligosaccharide, which is
part of a cell-surface molecule
that defines a particular blood group.
HO
CH 2 OH
CH 2 OH
O
CH 2 OH
HO
O
O
O
OH
O
OH
O
NH
NH
O
C O
C O
CH 3
CH 3
CH 3
HO
O
OH